[Mybatis] One of the cascading queries to a cascading query

introduction

When talking about resultMapelements earlier, [Mybatis] configures the resultMap element and resultType element of the mapping file . This element contains a sub-element association, Mybatiswhich is mainly used to process one-to-one association relationships.
associationelement, usually the following attributes can be configured

• property: Specifies the object property mapped to the entity class.
• column: Specifies the corresponding field in the table (that is, the column name returned by the query).
• javaType: Specifies the type mapped to the attribute of the entity object.
• select: Specifies the sub-SQL statement that introduces the nested query, this attribute is used for the nested query in the association map

Application scenario example: the relationship between a person and an ID card is one-to-one, that is, a person can only have one ID card, and an ID card can only correspond to one person.

Create project and database tables

Create a Spring integration Mybatis project [Spring] Spring integration Mybatis case

Create idcardtables and usertables

DROP TABLE IF EXISTS `idcard`;
CREATE TABLE `idcard` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `code` varchar(25) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `user` (
  `card_id` int(11) DEFAULT NULL,
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `card_id` (`card_id`),
  CONSTRAINT `card_id` FOREIGN KEY (`card_id`) REFERENCES `idcard` (`id`) ON DELETE SET NULL
) 

The project structure is as follows:

Create entity classes and database operation interfaces

IdCard class

public class IdCard {
    
    
    private int id;
    private String code;

    public int getId() {
    
    
        return id;
    }

    public void setId(int id) {
    
    
        this.id = id;
    }

    public String getCode() {
    
    
        return code;
    }

    public void setCode(String code) {
    
    
        this.code = code;
    }

    @Override
    public String toString() {
    
    
        return "Idcard{" +
                "id=" + id +
                ", code='" + code + '\'' +
                '}';
    }
}

User class

public class User {
    
    
    private int id;
    private String name;
    private int age;
    private IdCard idcard;

    public int getId() {
    
    
        return id;
    }

    public void setId(int id) {
    
    
        this.id = id;
    }

    public String getName() {
    
    
        return name;
    }

    public void setName(String name) {
    
    
        this.name = name;
    }

    public int getAge() {
    
    
        return age;
    }

    public void setAge(int age) {
    
    
        this.age = age;
    }

    public IdCard getIdcard() {
    
    
        return idcard;
    }

    public void setIdcard(IdCard idcard) {
    
    
        this.idcard = idcard;
    }

    @Override
    public String toString() {
    
    
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", age=" + age +
                ", idcard=" + idcard +
                '}';
    }
}

IdCardMapper

@Repository("idCardMapper")
@Mapper
public interface IdCardMapper {
    
    
    IdCard findCardById(int id);
}

UserMapper

@Repository("userMapper")
@Mapper
public interface UserMapper {
    
    
    List<User> findAllUser();
}

Create configuration file

mybatis-config.xml

<?xml version = "1.0" encoding = "UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0 //EN"
        "http//mybatis.org/dtd/mybatis-3-config.dtd" >
<configuration>
    <settings>
        <!--在使用MyBatis嵌套查询方式进行关联查询时，使用MyBatis的延迟加载可以在一定程度上提高查询效率-->
        <!--打开延迟加载的开关-->
        <setting name="lazyLoadingEnabled" value="true"/>
        <!--将积极加载改为按需加载-->
        <setting name="aggressiveLazyLoading" value="false"/>
    </settings>
    <mappers>
        <mapper resource="UserMapper.xml"/>
        <mapper resource="IdCardMapper.xml"/>
    </mappers>

</configuration>

IdCardMapper.xml

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.lucas.mybatis.mapper.IdCardMapper">
    <select id="findCardById" parameterType="Integer" resultType="com.lucas.mybatis.model.IdCard" >
        select  * from idcard where id=#{id}
    </select>

</mapper>

UserMapper.xml

method 1

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.lucas.mybatis.mapper.UserMapper">
    <!-- 定义结果集，column表示sql查询列,property表示映射到实体的属性 -->
    <resultMap type="com.lucas.mybatis.model.User" id="cardAndUser">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        <!-- 关联对象 -->
        <!--  使用select属性指定第二步调用的方法,并通过column指定传递的参数值，这个值是第一步的查询的数据 -->
        <association property="idcard" column="card_id"
                     javaType="com.lucas.mybatis.model.IdCard" select="com.lucas.mybatis.mapper.IdCardMapper.findCardById"/>
    </resultMap>
    <!-- 	第一步值只查询user表 -->
    <select id="findAllUser"   resultMap="cardAndUser">
			select * from user
	</select>

</mapper>

Method 2

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.lucas.mybatis.mapper.UserMapper">

    <resultMap type="com.lucas.mybatis.model.User" id="cardAndUser">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        <!-- 一对一级联查询-->
        <association property="idcard" javaType="com.lucas.mybatis.model.IdCard" column="card_id">
            <!-- idcard的id对应着user的card_id -->
            <id property="id" column="card_id"/>
            <result property="code" column="code"/>
        </association>
    </resultMap>
    <!-- resultMap指定使用上面定义的结果集,查询使用关联查询，查询列要和上面的column对应 -->
    <select id="findAllUser"  resultMap="cardAndUser">
		select user.*,idcard.code
		from `user`,idcard
		where user.card_id=idcard.id
		</select>
</mapper>

Method 3

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.lucas.mybatis.mapper.UserMapper">
   <select id="findAllUser" resultType="com.lucas.mybatis.model.UserPojo" >
          select user.*,idcard.code from `user`,idcard where user.card_id=idcard.id 
       </select>
</mapper>

UserPojokind

public class UserPojo {
    
    
    private int id;
    private String name;
    private int age;
    private String code;

    public int getId() {
    
    
        return id;
    }

    public void setId(int id) {
    
    
        this.id = id;
    }

    public String getName() {
    
    
        return name;
    }

    public void setName(String name) {
    
    
        this.name = name;
    }

    public int getAge() {
    
    
        return age;
    }

    public void setAge(int age) {
    
    
        this.age = age;
    }

    public String getCode() {
    
    
        return code;
    }

    public void setCode(String code) {
    
    
        this.code = code;
    }

    @Override
    public String toString() {
    
    
        return "UserPojo{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", age=" + age +
                ", code='" + code + '\'' +
                '}';
    }
}

add test code

public class App {
    
    
    public static void main(String[] args) {
    
    
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
        UserMapper userMapper = (UserMapper) applicationContext.getBean("userMapper");
       List<User> users = userMapper.findAllUser();
       for (User user :users){
    
    
           System.out.println(user);
       }
    }
}